Method and apparatus for transmitting lte waveforms in shared spectrum by carrier sensing

ABSTRACT

A method of operating a long term evolution (LTE) communication system on a shared frequency spectrum is disclosed. A user equipment (UE) is initialized on an LTE frequency band. A base station (eNB) monitors the shared frequency spectrum to determine if it is BUSY. The eNB transmits to the UE on the shared frequency spectrum if it is not BUSY. The eNB waits for a first time if it is BUSY and directs the UE to vacate the shared frequency spectrum after the first time.

BACKGROUND OF THE INVENTION

This application is a continuation of U.S. patent application Ser. No.15/888,928 filed Feb. 5, 2018, which is a continuation of U.S. patentapplication Ser. No. 15/227,406 filed Aug. 3, 2016, now U.S. Pat. No.9,888,389, which is a continuation of U.S. patent application Ser. No.14/718,593 filed May 21, 2015, now U.S. Pat. No. 9,532,230, and claimsthe benefit under 35 U.S.C. § 119(e) of Provisional Appl. No.62/008,032, filed Jun. 5, 2014, which is incorporated herein byreference in its entirety.

In most countries, access to radio frequency spectrum is tightlyregulated through government agencies such as the Federal CommunicationsCommission (FCC) in the United States or the European Commission in theEuropean Union. Like any other natural resource, the frequencies thatmake up the radio spectrum need to be shared among its users. Parts ofthe radio spectrum, so-called bands, are thus either licensed toindividual users, such as mobile operators, or shared among many usersas is the case with WiFi or Bluetooth which operate in unlicensed bands.In addition, certain hybrid models exist where licensed spectrum isgranted to a primary user, for instance for naval radar applications,who has the highest priority. In addition, secondary users are allowedto use the licensed band during periods of inactivity during which theprimary user does not transmit waveforms in the band underconsideration. These secondary users may have a different priority. Forexample, a given frequency band licensed to a primary user could be usedby the public safety community for mission critical communications. Inthis case, commercial users could be allowed to use such a band and onlyif both the primary and the secondary user of higher priority, i.e. apublic safety user, do not occupy the band. Such policy based spectrumusage is sometimes referred to as Authorized Shared Access (ASA). Fromthis perspective, there is no need to distinguish between unlicensed andauthorized shared access as the same techniques can be used to ensurefairness and policy compliance whenever a band is used by many users.

In the above example of authorized shared access, spectrum sharing canbe facilitated by dynamic schemes, sometimes referred to aslisten-before-talk (LBT) schemes, as well as semi-static schemes, suchas through geolocation databases (GLDB). Such databases, for example,can map frequency usage of certain bands to geographic areas or times ofday. Due to the time it takes to update and propagate these databases toall participating users, they cannot change dynamically. As the namesuggests, LBT schemes are more dynamic and do not rely onsemi-statically configured databases. Rather, a secondary user has toensure that the primary user or other user of equal priority is notinterfered with by its transmission. Two well-known examples are radaravoidance and Carrier Sense Multiple Access with Collision Avoidance(CSMA/CA) in IEEE 802.11 Wireless Local Area Networks (WLANs). Theformer applies to the case when there is a primary user, to which asecondary user must grant priority. Since the secondary must ceasetransmission when it detects a military, meteorological or automotiveradar waveform, it is often referred to as Dynamic Frequency Selection(DFS). In other words, the secondary user vacates a given band orchannel (channels are further subdivisions of bands) upon detection of aprimary user and tries to transmit on a different band or channel givingrise to the name dynamic frequency selection. Similarly, in the case ofCSMA/CA, when the transmitter detects an ongoing transmission of equalpriority, it chooses not to transmit in order to try again at a laterpoint in time. Hence the name carrier sense multiple access withcollision avoidance. The two main differences between DFS and CSMA/CA,therefore, are the time scale at which the sensing occurs and the actionthe transmitter takes when an on-going transmission is detected. Forexample, a DFS transmitter will always have to switch channels/bands inorder to vacate the current one for the primary user, whereas a CSMA/CAtransmitter may or may not switch the channel. This is because inCSMA/CA the radio resources are shared among users of equal priority,and it is considered a multiple access scheme. With DFS, however, theprimary user has higher priority. Consequently, in order to guaranteecompetitive latencies of CSMA/CA schemes, the carrier sensing (CS) andcollision avoidance (CA) occurs in the order of tens of microseconds(μs) whereas DFS may take seconds.

CS/CA multiple access schemes stand in stark contrast to other commonmultiple access techniques such as Time Division Multiple Access (TDMA),Frequency Division Multiple Access (FDMA), Code Division Multiple Access(CDMA), or Orthogonal Frequency Division Multiple Access (OFDMA) due tothe opportunistic random access nature by which the medium is shared.TDMA and FDMA in the Global System for Mobile Communications (GSM), CDMAin the Universal Mobile Telecommunications System (UMTS), and OrthogonalFrequency-Division Multiple Access (OFDMA) in the 3rd GenerationPartnership Project (3GPP) Long Term Evolution (LTE)) try toorthogonalize the available resources to share them among multipleusers. Orthogonal operation, however, requires precise coordinationthrough predefined rules or a dynamic scheduler which assigns resourcesto particular users for a given period of time in a given part of theradio frequency spectrum such that collisions are inherently prevented.This makes it particularly challenging to operate them in radioresources which are shared by means of CS/CA multiple access schemes,since users following those kinds of protocols and procedures would loseto users following predefined schedules or radio resource assignmentsaccording to their protocols and procedures when competing for theavailable radio resources.

In LTE a base station is known as an evolved NodeB (eNodeB/eNB) and isin full control of the Radio Resource Management (RRM) of a cell underits control. An Evolved Universal Terrestrial Radio Access Network(E-UTRAN) generally comprises of many eNodeBs each with its own RRMfunction. A subset of these eNodeBs can coordinate their RRM through theX2 Application Protocol (X2AP) which is defined on the X2 interfaceconnecting two eNodeBs. Similarly, each eNodeB is connected to one ormore of the Mobility Management Entities (MMES) in the Core Network (CN)via the S1 interface on which the S1 Application Protocol (S1AP) isdefined. The S1AP can be used for RRM coordination as well. RRMinterfaces are an integral part of cellular communications as they allowimportant functions such as interference coordination, mobility, or evenSelf-Organizing Networks (SONs).

FIG. 1 is an exemplary wireless telecommunications network of the priorart. The illustrative telecommunications network includes primary eNodeB110 operating in primary cell (PCell) 100 and eNodeBs 112, 114, 116, and118 operating in secondary cells (SCell₁ through SCell₄) 102, 104, 106,and 108, respectively. A handset or other user equipment (UE) 120 isshown in communication with eNodeB 110 of PCell 100. UE 120 may also bein communication with one or more eNodeBs of the secondary cells. Here,SCell is a logical concept. For example, eNodeB 110 could operate aplurality of SCells 102 through 108.

In addition, eNodeB 110 is in control of the radio resources in its cell100 by means of the Radio Resource Control (RRC) protocol as well as themultiple access of the users connected to its cell by means of theMedium Access Control (MAC) protocol. The RRC protocol, for instance,configures the carriers of which a User Equipment (UE) can transmit andreceive data and up to five so-called Component Carriers (CCs) can beconfigured per UE in LTE Advanced (LTE-A). Similarly, the MAC protocolin conjunction with the RRC protocol controls how and when the UE canuse the available radio resources to transmit or receive data on aconfigured carrier. LTE Release 10 introduces a feature called CarrierAggregation in which a UE can be configured with one primary cell(PCell) and up to four secondary cells (SCells). A PCell can only bechanged through a handover, whereas SCells are configured through RRCsignaling. In particular, a UE is not expected to receive systeminformation by decoding the Physical Broadcast Channel (PBCH) on aSecondary Component Carrier (SCC) or to monitor the common search spaceof an SCell to receive Physical Downlink Control Channels (PDCCHs) whoseCRC is scrambled by the SI-RNTI in order to receive System Information(SI) on the Downlink Shared Channel (DL-SCH). Moreover, the UE mayassume that the System Frame Number (SFN) on all SCCs is aligned withthe SFN of the Primary Component Carrier (PCC).

CA does not define Radio Link Monitoring (RLM) of an SCell. As such,there is no specified means for the UE Physical layer (PHY) to indicatea Radio Link Failure (RLF) to the UE higher layers through the MAClayer. This is because in the Evolved Universal Terrestrial Radio Access(E-UTRA) one can always rely on the connectivity provided by the PCellwhich provides robustness through RLM and other fallback procedures.Alternatively, one may think of SCells as supplementary serving cellswhich can be activated in case additional capacity is needed for datacommunication with the UE. To this end, the MAC layer can activateconfigured SCells through a MAC Control Element (CE). An SCellactivation can take between 8-30 ms depending on the synchronizationstatus of the UE with respect to that SCC. An RRC reconfiguration of anSCell would take significantly longer, especially when the UE needs toperform an inter-frequency measurement. The eNodeB may thus configure aUE to periodically measure the Reference Signal Received Power (RSRP) ofcertain cells on certain carriers and to report the measurements eitherperiodically or triggered through configurable offsets and thresholds.In the 3GPP Long Term Evolution this is achieved through RRC signalingof measurement objects and configurations. If measurements are readilyavailable at the eNodeB, an RRC reconfiguration of an SCell or PCell canbe dramatically reduced in latency from seconds to tens or hundreds ofmilliseconds. Note that while the eNodeB can only activate cells thatare already configured as SCells, it can configure a UE to measure theRSRP on any cell. On the other hand, the eNodeB can use the measurementreport of any cell to either activate a cell, as in the case of SCellactivations, or to RRC reconfigure the UE to add/remove SCells or evento change the PCell.

Once a PCell or SCell is activated, the eNodeB MAC scheduler assignsdownlink (DL) and uplink (UL) grants to a UE for downlink and uplinktransmissions on the Physical Downlink Shared Channel (PDSCH) andPhysical Uplink Shared Channel (PUSCH), respectively. In the downlinkdirection, a grant received in the Downlink Control Information (DCI) insubframe n schedules a corresponding PDSCH transmission in the samesubframe whereas in the uplink, it schedules PUSCH transmissions insubframe n+k, where k>0 is determined through pre-specified rules.

It is worth reiterating that the E-UTRAN, in particular, the eNodeB, isin full control of all radio resources at least for UEs in RRC CONNECTEDmode and that, with the exception of the Physical Random Access Channel(PRACH), it controls all transmissions in both the uplink and downlinkdirection including resource assignment in time, frequency, or any othermeans such as CDMA as well as timing or power control of a transmission.

Even though the RRM function resides in the eNodeB, which in turncontrols all radio resources through RRC, it relies on the UE todiscover cells and report associated measurements. To this end, in LTEReleases 8 through 11, the eNodeB transmits a Primary SynchronizationSignal (PSS), a Secondary Synchronization Signal (SSS), and aCell-specific Reference Signal (CRS) in each radio frame. The PSS andSSS each occupy one OFDM symbol per half-frame whereas the CRS istransmitted in each subframe of a radio frame thus allowing a UE todiscover and measure cells within a measurement window of 6 ms withouta-priori knowing the timing of a given cell. Furthermore, to supportinter-frequency measurements in Time Division Duplex (TDD) systems whenthe UL/DL configuration of a cell may not be known to a UE or to supportmeasurement restrictions introduced in LTE Rel. 10 for the purpose ofenhanced Inter-cell Interference Coordination (eICIC), a UE must be ableto discover cells in just one subframe and potentially the DwPTS regionof a special subframe. In order to facilitate energy savings andinterference reduction, LTE Release 12 introduces “discovery bursts”comprising PSS, SSS, and CRS transmissions and, if configured, ChannelState Information Reference Signals (CSI-RS) for transmission point (TP)identification in shared cell ID scenarios. For example, multiple TPsmay share the same physical cell ID and may only be discerned by theirrespective CSI-RS resource element (RE) configuration. PSS, SSS, CRS andCSI-RS (if configured) make up the Discovery Reference Signals (DRS) andare transmitted during DRS occasions. DRS occasions are similar to LTERelease 9 Positioning Reference Signal (PRS) occasions in that they havea configured or specified length (i.e. number of subframes) andperiodicity. Ideally, the length of a DRS occasion does not exceed theUE measurement window of 6 ms and could be as short as one subframe.Reasonable periodicities for DRS occasions are hundreds of millisecondsand DRS bursts can be thought of as beacons in other wirelesscommunication systems such as CSMA/CA.

BRIEF SUMMARY OF THE INVENTION

In a first preferred embodiment of the present invention, there isdisclosed a method of operating a long term evolution (LTE)communication system on a shared frequency spectrum. A base station(eNB) initializes a user equipment (UE) on an LTE frequency band. TheeNB monitors the shared frequency spectrum to determine if it is BUSY.The eNB transmits to the UE on the shared frequency spectrum if it isnot BUSY. The eNB waits for a first time if it is BUSY. The eNB directsthe UE to vacate the shared frequency spectrum after the first time.

In a second preferred embodiment of the present invention, there isdisclosed a method of operating a long term evolution (LTE)communication system on a shared frequency spectrum. A base station(eNB) initializes a user equipment (UE) on an LTE frequency band. The UEmonitors the shared frequency spectrum to determine if it is BUSY. TheUE transmits to the eNB on the shared frequency spectrum if it is notBUSY. The UE waits for a first time if it is BUSY. The UE reports theBUSY condition to the eNB after the first time.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is a diagram of a long term evolution (LTE) communication systemof the prior art;

FIG. 2 is a flow diagram showing operation of a long term evolution(LTE) communication system on an authorized shared access (ASA)frequency spectrum;

FIG. 3 is a diagram showing communication between a user equipment (UE)and a base station (eNB) according to the present invention;

FIG. 4A is a flow diagram showing downlink operation of a long termevolution (LTE) communication system on a Carrier Sense Multiple Accesswith Collision Avoidance (CSMA/CA) frequency spectrum according to thepresent invention; and

FIG. 4B is a flow diagram showing uplink operation of a long termevolution (LTE) communication system on a Carrier Sense Multiple Accesswith Collision Avoidance (CSMA/CA) frequency spectrum according to thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to an apparatus and method ofoperation of an Orthogonal Frequency-Division Multiple Access (OFDMA)cellular communications system such as the 3GPP Long Term Evolution(LTE) in radio frequencies shared with a primary transceiver. Theprimary transceiver may be a naval, automotive radio, or othertransceiver of higher priority. Many modifications and other embodimentsof this invention will come to mind to one skilled in the art to whichthe invention pertains having the benefit of the teachings presented inthe descriptions and the associated drawings. Therefore, it is to beunderstood that the present invention is not limited to the specificembodiments disclosed. Although specific terms are employed herein, theyare used in a generic and descriptive sense only and not for purposes oflimitation.

-   The following abbreviations are used throughout the instant    specification.    -   ASA: Authorized Shared Access    -   eNB: evolved Node B or base station    -   UE: User Equipment    -   CQI: Channel Quality Indicator    -   CRS: Cell-specific Reference Signal    -   CSI: Channel State Information    -   CSI-RS: Channel State Information Reference Signal    -   CSMA/CA: Carrier Sense Multiple Access with Collision Avoidance    -   DCI: Downlink Control Information    -   DFS: Dynamic Frequency Selection    -   DRS: Discovery Reference Signal    -   DL: DownLink    -   DwPTS: Downlink Pilot Time Slot    -   E-UTRAN: Evolved Universal Terrestrial Radio Access Network    -   LBT: Listen Before Talk    -   LTE: Long Term Evolution    -   MAC: Medium Access Control protocol    -   MIMO: Multiple-Input Multiple-Output    -   OFDMA: Orthogonal Frequency Division Multiple Access    -   OOR: Out Of Range    -   PBCH: Physical Broadcast Channel    -   PCell: Primary Cell    -   PCFICH: Physical Control Format Indicator Channel    -   PDCCH: Physical Downlink Control Channel    -   PDSCH: Physical Downlink Shared Channel    -   PHICH: Physical Hybrid ARQ Indicator Channel    -   PMCH: Physical Multicast Channel    -   PSS: Primary Synchronization Signal    -   PUCCH: Physical Uplink Control Channel    -   PUSCH: Physical Uplink Shared Channel    -   RI: Rank Indicator    -   RRC: Radio Resource Control    -   RRM: Radio Resource Management    -   RSRP: Reference Signal Received Power    -   SCell: Secondary Cell    -   SRS: Sounding Reference Signal    -   SSS: Secondary Synchronization Signal    -   TDD: Time Division Duplex    -   TRS: Tracking Reference Signal    -   UL: UpLink

Dynamic Frequency Selection (DFS)

The 3GPP Long Term Evolution (LTE) communications standard cannot bereadily deployed in shared access spectrum. This is because the radioresource management function resides in, and the radio resources aresolely controlled by, the eNodeBs in the network. Dynamic FrequencySelection (DFS) schemes typically allow sufficient time (e.g. severalseconds) to change a frequency band or carrier upon detection of aprimary user. Thus, handover based RRC signaling and SCell activation ordeactivation under MAC control are sufficient to vacate a band for aprimary user. The 3GPP LTE communications standard currently lacksprotocols, procedures and measurements that would let a UE take anyaction in case a primary user is detected on a carrier on which the UEis configured to transmit data. Furthermore, mobility control in LTE isfully controlled by the eNodeB, although other wireless cellularcommunications standards do allow UEs to initiate handovers. Mobilityhere incorporates the case of load balancing where the eNodeB may add orremove SCells or change the PCell for stationary UEs. For both ASA basedschemes with a primary user and CSMA/CA based schemes without a primaryuser, so called “hidden stations” may exist. Hidden stations aretransmitters such as primary users, whose transmissions can only bedetected at the receiving end of a communications link which shares thewireless medium. In LTE, for example, only the UE may detect waveformstransmitted from a “hidden station” whereas the eNodeB is completelyoblivious to the existence of the hidden station.

Referring to FIG. 2, there is a diagram showing operation a firstembodiment of the present invention. A UE is initialized at step 200 tooperate in conjunction with a PCell on an LTE band. An ASA band isconfigured and operated as a regular LTE band by the eNodeB, and the UEoperates on the ASA band 202. UEs are barred from camping on cellsoperating in the ASA band through existing means, such as barringthrough broadcast of system information. Consequently, all UEs connectedon the ASA band are in RRC CONNECTED mode and thus under full control ofan eNodeB. The eNodeB configures all UEs connected on the ASA band toperform RRM measurements 204 as per existing LTE specifications (e.g.Releases 8 through 12). DFS is supported by each UE throughnon-standardized (proprietary) implementations. If a UE detects a hiddenstation (from the UE perspective, all primary users are hidden stations)206, the UE Physical Layer (PHY) indicates to the higher layers of itsprotocol stack to trigger an RRM measurement report as per existing LTERel. 8/9/10/11/12 procedures. Through specification, a “DFS event,” forexample, an RRM measurement report triggered through thenon-standardized (proprietary) DFS function at the UE would be tied to aspecific value of the RRC Information Element (IE) RSRP-Range. Forexample, a DFS event could be indicated by the lowest value in the RRCIE RSRP-Range and could be thought of as an Out-of-Range (OOR)indication. The UE would use existing RRM measurement reportingprocedures to report the DFS event (i.e., the RSRP measurement reportwith the OOR indicator signifying the DFS event) to the eNodeB 208. TheeNodeB RRM function would re-interpret the RSRP measurement report as aDFS/OOR event as per the standardized linkage and subsequently, tovacate the ASA band 210 for the primary user, and would reconfigure theUE via existing RRC signaling 212. Such RRC signaling encompasseshandovers in the case of PCells or SCell reconfigurations in the case ofSCells. Alternatively, if the RRM function at the eNodeB believes theASA band needs to be only temporarily vacated for the primary user, itcould simply let the sCellDeactivationTimer at the UE expire, or it cansend a deactivation command in a MAC control element in order todeactivate an SCell configured on the ASA band. 3GPP LTE specificationswould introduce performance requirements that can be used to test UEs ifthey report DFS/OOR events according to requirements put forward byregulatory bodies worldwide for each ASA band but no new measurementswould be defined in the specifications to support DFS in 3GPP LTE.

In another embodiment of the present invention, instead ofre-interpreting an existing measurement report as DFS/OOR event, a newmeasurement report and associated procedures are defined specificallyfor the purpose of indicating to the E-UTRAN the existence of a hiddenstation or primary user. All UEs connected to cells on an ASA band wouldbe configured to measure and report this new DFS measurement. The eNodeBRRC layer can configure UEs to report the DFS measurement eitherperiodically or triggered, or periodically triggered. The eNodeB wouldconfigure measurement events and associated thresholds and offsets tocontrol the DFS measurement reporting of UEs connected on ASA bands. Theexact measurement procedure would thus be determined by specification.However, the actions taken by the network could be similar to those inthe previous embodiment above to include UE handover, SCellreconfiguration, and SCell deactivation. Reporting a measurement ratherthan binary information would let the eNodeB RRM function learn fromhistorical data and let it apply its own threshold for improvedprotection of the primary user. Since the eNodeB can analyze and combineDFS measurements from various UEs connected to it, the decision toselect a different carrier for a given UE ultimately resides at theeNodeB. However, if the decision is made at each UE, the network wouldhave to follow whatever a UE indicates in order to guarantee protectionof a potential primary user.

Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA)

Turning now to FIG. 3, there is a diagram showing communication betweenUE 300 and eNodeB 320 according to the present invention. UE 300 may bea cell phone, computer, or other wireless network device. UE 300includes a processor 306 coupled to a memory 304 and a transceiver 310.Processor 306 may include several processors adapted to variousoperational tasks of the UE including signal processing and channelmeasurement and computation. The memory stores application software 302that the processor may execute as directed by the user as well asoperating instructions for the UE. Processor 306 is also coupled toinput/output (I/O) circuitry 308, which may include a microphone,speaker, display, and related software. Transceiver 310 includesreceiver 312 and transmitter 314, suitable for wireless communicationwith eNodeB 320. Transceiver 310 typically communicates with eNB 320over various communication channels. For example, transceiver 310 sendsuplink information to eNodeB 320 over physical uplink control channelPUCCH and physical uplink shared channel PUSCH. Correspondingly,transceiver 310 receives downlink information from eNodeB 320 overphysical downlink control channel PDCCH and physical downlink sharedchannel PDSCH.

Base station 320 includes a processor 326 coupled to a memory 324, asymbol processing circuit 328, and a transceiver 330 via bus 336.Processor 326 and symbol processing circuit 328 may include severalprocessors adapted to various operational tasks including signalprocessing and channel measurement and computation. The memory storesapplication software 322 that the processor may execute for specificusers as well as operating instructions for eNodeB 320. Transceiver 330includes receiver 332 and transmitter 334, suitable for wirelesscommunication with UE 300. Transceiver 330 typically communicates withUE 300 over various communication channels. For example, transceiver 330sends downlink information to UE 300 over physical downlink controlchannel PDCCH and physical downlink shared channel PDSCH. Transceiver330 also sends special downlink information to UE 300 over physicalbroadcast channel PBCH, physical hybrid ARQ indicator channel PHICH,physical control format indicator channel PCFICH, and physical multicastchannel PMCH. Correspondingly, transceiver 330 receives uplinkinformation from UE 300 over physical uplink control channel PUCCH andphysical uplink shared channel PUSCH.

According to the present invention, E-UTRAN cells such as eNodeB 320 maybe deployed in unlicensed or ASA bands where LTE user equipment sharesthe radio resources with other users of equal priority but which followstrict Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA)procedures/protocols. There is a fundamental problem in that the 3GPPLong Term Evolution was specifically designed to operate in licensedspectrum.

Referring now to FIG. 4A, in the downlink direction the situation issimilar to DFS as explained with reference to FIG. 2. Here, CSMA/CA isimplemented as a non-standardized proprietary function according to thepresent invention. The UE is initialized on an LTE band 400. The eNodeBmonitors the CSMA/CA band 402. If the eNodeB senses an ongoingtransmission 404, it does not transmit any downlink channels. The eNodeBmonitors a timeout reference 408 and continues to monitor the CSMA/CAband 402. If the ongoing transmission ends before the timeout reference408, the eNodeB transmits to the UE on the CSMA/CA band 406. Otherwise,if the timeout reference expires, RRC signaling directs the UE to vacatethe CSMA/CA band 410 and initiates a handover 412.

The eNodeB may, however, have to transmit some signals without regard towhether an ongoing transmission is detected. The eNodeB transmitsDiscovery Reference Signal (DRS) bursts with a periodicity in the orderof hundreds of milliseconds. The DRS burst may just be one subframe andcomprises at least PSS, SSS, and CRS to allow UEs to discover the celland perform measurements. For shared cell ID scenarios, CSI-RS may alsobe transmitted during a DRS occasion. The periodic PSS/SSS transmissionsalso let UEs obtain coarse time and frequency synchronization with thatcell. At the network side, the DRS based RRM measurement reports let theeNodeB decide whether to configure a cell on a certain unlicensed or ASAband for a given UE. In addition to

DRS, the eNodeB needs to periodically transmit some kind of TrackingReference Signal (TRS) with a much smaller periodicity than that of DRS,such as 5 ms or 10 ms. The TRS waveforms let UEs perform Automatic GainControl (AGC) and fine time and frequency synchronization (“tracking”).Such TRS waveforms may be based on existing CRS waveforms. This wouldhave the additional benefit that it could be used for channel stateinformation acquisition in case of CRS-based transmission modes.Additionally, the eNodeB may periodically transmit Channel StateInformation Reference Signals (CSI-RS) to allow channel stateinformation acquisition at the UE for CSI-RS based transmission modes.UEs would be configured for CSI measurement and reporting in accordancewith the CSI transmissions at the eNodeB.

Referring back to FIG. 3, it may be preferable not to use some downlinkchannels with CSMA/CA. For example, the Physical Broadcast Channel(PBCH) would not be transmitted in a cell on an unlicensed or ASA band.Accordingly, UEs would not be able to camp on such a cell. Similarly,system information would also not be transmitted. Such cells can thusonly be configured as SCells and PCells would always be configured onlicensed spectrum. It may also be beneficial not to transmit thePhysical Hybrid ARQ Indicator CHannel (PHICH) in unlicensed or ASAspectrum. Alternatively, UL grants transmitted in DCI could serve asimplicit ACK/NACK indication by scheduling retransmissions of previousUL grants. The Physical Control Format Indicator Channel (PCFICH) may ormay not be transmitted in unlicensed or ASA spectrum. If extended PHICHduration is configured, the Control Format Indicator (CFI) is knownthrough specification. Similarly, the PCFICH is not needed for PDSCHtransmissions in transmission mode 10 (TM10) scheduled by an EnhancedPhysical Downlink Control Channel (EPDCCH). And for cross-carrierscheduled PDSCH transmissions the CFI is known through configuration. Onthe other hand, since the PCFICH is transmitted in the same subframe asa PDCCH it could be transmitted whenever a PDCCH is transmitted.Finally, since the Physical Multicast Channel (PMCH) is scheduledsemi-statically by the MBMS Coordination Entity (MCE) on reservedresources, it may be beneficial not to transmit the PMCH in unlicensedor ASA spectrum. Otherwise, for unicast downlink transmissions, when theCSMA/CA function at the eNodeB indicates that a given subframe can beused for (E)PDCCH or PDSCH transmissions, the eNodeB transmits as perLTE Release 12. In one embodiment, the CSMA/CA function at the eNodeBreturns a binary indication. If the CSMA/CA function for a given cell ona given carrier indicates BUSY, the eNodeB does not transmit (E)PDCCH orPDSCH to any UE. The eNodeB may still transmit other signals or channelsas per the above recommendations. Alternatively, if the CSMA/CA functionfor a given cell on a given carrier indicates IDLE, the eNodeB maytransmit (E)PDCCH and/or PDSCH transmissions, the eNodeB transmits asper LTE Release 12.

Referring to FIG. 4B, uplink operation on CSMA/CA bands is similar todownlink operation. The UE is initialized on an LTE band 400. The UEmonitors the CSMA/CA band 420. If the UE senses an ongoing transmission422, it does not transmit any uplink channels. The UE monitors a timeoutreference 426 and continues to monitor the CSMA/CA band 420. If theongoing transmission ends before the timeout reference 426, the UEtransmits to the eNodeB on the CSMA/CA band 424. Otherwise, if thetimeout reference expires, the UE sends a BUSY report to the eNodeB 428.RRC signaling directs the UE to vacate the CSMA/CA band 430 andinitiates a handover 432.

When the CSMA/CA function at the UE indicates that a given subframecannot be used for uplink transmissions, it may be beneficial to dropany configured Sounding Reference Signal (SRS) transmission in order tonot interfere with the ongoing transmission. It may also be beneficialnot to transmit the Physical Uplink Control Channel (PUCCH) inunlicensed or ASA spectrum. In this case, the PUCCH is transmitted onthe PCell in licensed spectrum only. If PUCCH transmissions are allowedin unlicensed or ASA spectrum, several UE behaviors are envisioned.

In one case, the UE follows existing UE procedures for PUCCHtransmissions independent of the indication of the CSMA/CA function atthe UE for the subframe for which the PUCCH transmission is scheduled.Collisions with on-going transmissions cannot be avoided in general andthe PUCCH may not be properly received at the eNodeB.

Alternatively, the UE could base any PUCCH transmissions on theindication of the CSMA/CA function at the UE for the subframe for whichthe PUCCH transmission is scheduled. If the CSMA/CA function at the UEindicates BUSY, the UE does not transmit on the PUCCH in the subframeunder consideration. Otherwise, if the CSMA/CA function at the UEindicates IDLE, the UE transmits the PUCCH as scheduled.

The same principles may be applied to the Physical Uplink Shared Channel(PUSCH). In one embodiment, the UE follows existing UE procedures forPUSCH transmissions independent of the indication of the CSMA/CAfunction at the UE for the subframe for which the PUSCH transmission isscheduled. Collisions with on-going transmissions cannot be avoided ingeneral and the PUSCH may not be properly received at the eNodeB.

Alternatively, the UE could base any PUSCH transmissions on theindication of the CSMA/CA function at the UE for the subframe for whichthe PUSCH transmission is scheduled. If the CSMA/CA function at the UEindicates BUSY, the UE does not transmit on the PUSCH in the subframeunder consideration. Otherwise, if the CSMA/CA function at the UEindicates IDLE, the UE transmits the PUSCH as scheduled.

Similar to the case of DFS, hidden stations must be considered. Theabove solutions for PUSCH and PUCCH transmissions are concerned with theUE behavior in case the CSMA/CA function at the UE indicates BUSY forthe subframe for which the PUSCH/PUCCH transmission is scheduled. Incase of a hidden station whose waveform is detectable at the UE but notat the eNodeB, the eNodeB may continue scheduling that UE. In case theUE follows regular LTE Rel. 12 operation, this would result indeteriorated performance for both the eNodeB-to-UE link as well as forthe link to/from the hidden station, as the respective transmissionswould continue to collide potentially creating excessive interferencesuch that reliable communication is no longer feasible or at least,acceptable Quality-of-Service (QoS) could no longer be provided. Theopposite case, where the UE does not transmit on PUSCH or PUCCH in asubframe if the CSMA/CA function at the UE indicates BUSY, would equallydeteriorate performance due to the dropped packages and HARQ ACK/NACKtransmissions in BUSY subframes. In theory, the aforementioned DFSschemes could be reused to allow the UE to inform an eNodeB about theBUSY state of a cell or carrier such that the eNodeB MAC (or RRC) layercould take actions to schedule the UE on a different CC in order toprevent further collisions. In other words, instead of the “DFS event”triggered by the DFS function, the CSMA/CA function would indicate BUSYbut otherwise the procedures could be reused. Recall, however, that thetime scales for DFS are generally much larger than for LBT as in thecase of CSMA/CA. Thus, the present invention provides separateprocedures to address hidden stations in the case of CSMA/CA.

An objective of the present invention is to let the UE higher layersinform the eNodeB higher layers about the indication of the UE CSMA/CAfunction in subframes in which the UE is scheduled for uplinktransmissions. Since the UE can always follow existing LTE Rel. 12specifications in case the UE CSMA/CA function indicates IDLE, thisstate is not signaled to the eNodeB higher layers. Thus, severalembodiments of the present invention provide actions the eNodeB higherlayers, such as the eNodeB MAC scheduler, may take in a subframe forwhich a PUSCH or PUCCH transmission is scheduled and the UE CSMA/CAfunction indicates BUSY.

Since overall system performance and particularly the perceived userthroughput at the UE are maximized the faster the eNodeB can take actionby avoiding scheduling the UE on a carrier occupied by a hidden station,it is preferable to either use PHY or MAC layer mechanisms whereby theformer have lower latency than the latter. First, in order to reducelatencies, it is assumed that the UE is already configured with up tofive serving cells (FIG. 1) on corresponding component carriers.According to the present invention, the serving cells are ordered inascending order based on the ServCellIndex configured through RRCsignaling, however, other orderings and addressing mechanisms are notprecluded. Then, the four serving cells, excluding the PCell, areassigned the symbols {00,01,10,11}, such that the serving cell (SCell)with the lowest ServCellIndex corresponds to 00, the serving cell(SCell) with the second lowest ServCellIndex corresponds to 01, and soforth. If less than four SCells are configured the unused symbols, e.g.,{01,10,11} in case a single SCell is configured, are reserved. Othermappings are not precluded as they do not alter the invention. In orderto guarantee lowest latencies, L1 (PHY) signaling is introduced toinform the eNodeB higher layers about the BUSY indication from the UECSMA/CA function in a subframe for which a PUSCH or PUCCH transmissionis scheduled. More precisely, a new PUCCH format is introduced which isalways transmitted on the PCell. The new PUCCH format is identical tothe existing PUCCH format 1b, however, instead of representing ACK/ACK,ACK/NACK, NACK/ACK, and NACK/NACK/DTX, the QPSK symbols encode the fourserving cell indices {00,01,10,11}. For purposes of illustration thisnew PUCCH format is referred to as format 1c. The eNodeB receiver maydistinguish between PUCCH formats 1b and 1c through Code DivisionMultiplexing such that the two PUCCH formats can share the same time andfrequency resources. Alternatively, the new PUCCH format can have itsown time and frequency resources of the PUCCH region. If PUCCH capacityis not an issue, as is the case for small cells, CDM is preferred forimproved spectral efficiency. In case the CSMA/CA function at the UEindicates BUSY in a subframe for which a PUSCH or PUCCH transmission isscheduled the UE does not transmit the PUSCH or PUCCH as scheduled butrather indicates to the eNodeB the BUSY indication via a PUCCH format 1ctransmission on the PCell. Several UE behaviors are envisioned, all ofwhich assume that the eNodeB schedules only one SCell at a time in orderto prevent any ambiguities at the eNodeB when PUCCH format 1c isreceived.

In one embodiment, the PUCCH format 1c indicates on which serving cellthe BUSY indication occurred. For example, the eNodeB may schedule anuplink transmission in subframe n+k, k>0 via an UL grant in DCI receivedin subframe n. Shortly before the uplink transmission is scheduled tooccur, the CSMA/CA function at the UE begins to sense the medium andindicates to the UE higher layers if it is IDLE or BUSY. If IDLE isindicated, the UE proceeds with the scheduled transmissions as per thereceived DCI. If BUSY is indicated, the UE ignores the DCI schedulingthe uplink transmission under consideration and instead sends a PUCCHformat 1c on the PCell encoding in the QPSK symbol the serving cell onwhich the collision occurred.

Since the eNodeB expected the PUSCH or PUCCH transmission on aparticular serving cell, the PUCCH format 1c transmission does notreally convey additional information to the eNodeB higher layers. Thus,in a different embodiment, the CSMA/CA function at the UE senses allconfigured serving cells prior to a scheduled uplink transmission. IfIDLE is indicated for the serving cell on which the transmission isscheduled, the UE proceeds with the scheduled transmissions as per thereceived DCI. If BUSY is indicated, the UE ignores the DCI schedulingthe uplink transmission under consideration and instead sends a PUCCHformat 1c on the PCell encoding in the QPSK symbol a serving cell onwhich the CSMA/CA function at the UE indicated IDLE. This, does notguarantee that the corresponding serving cell is IDLE at a futuresubframe n+k₂, k₂>k, but at least the eNodeB does not continuescheduling uplink transmissions on the same serving cell.

Introducing the new PUCCH format 1c requires the eNodeB receiver tomonitor for the new PUCCH format. Accordingly, MAC layer procedures maybe preferable over the aforementioned PHY procedures. Sending MACcontrol elements, however, requires the UE to have available uplinkresources in addition to the ones it has to leave unused by nottransmitting PUCCH or PUSCH because the medium is BUSY. Moreover, thetime to prepare the PUSCH transmission carrying the MAC CE may takelonger such that the carrier sensing has to occur much earlier than inthe case of a new PUCCH format increasing the probability that theCSMA/CA function at the UE indicates IDLE but the medium is BUSY duringsubframe n+k. Latencies would be further increased if the UE has to senda Scheduling Request (SR) in order to transmit the MAC CE. Nevertheless,MAC layer procedures may still have their merits. For example, one wouldno longer need the restriction that only a single SCell is scheduled ata time. Rather, one octet (8 bits) in a MAC CE may be used to encode allfour SCells simultaneously. Up to four serving (SCells) are againordered in ascending order based on the ServCellIndex and represented by{00,01,10,11}, i.e., the serving cell (SCell) with the lowestServCellIndex corresponds to 00, the serving cell (SCell) with thesecond lowest ServCellIndex corresponds to 01, and so forth. Moreover,the 8 bits in an octet of a MAC CE correspond to the four SCells throughthe following mapping. The first two bits correspond to the serving cellrepresented by {00}, the third and fourth bit correspond to the servingcell represented by {01}, the fifth and sixth bit correspond to theserving cell represented by {10}, and the last two bits correspond tothe serving cell represented by {11}, although other mappings andorderings are not precluded. If the bits at a position correspond to theposition itself, this indicates that the corresponding serving cell wasindicated as IDLE. Otherwise, the indication was BUSY and the two bitsindicate to which serving cell the eNodeB should switch. In other words,the bit position in the octet encodes for which serving cell the bits atthat position belong and the bits themselves, encode the sameinformation transmitted on the PUCCH format 1c above for a single cell.For instance, the octet {00010011} means that the first, second, andforth serving cell were IDLE whereas transmissions on the third servingcell should be transmitted on the first serving cell.

Still further, while numerous examples have thus been provided, oneskilled in the art should recognize that various modifications,substitutions, or alterations may be made to the described embodimentswhile still falling within the inventive scope as defined by thefollowing claims. Other combinations will be readily apparent to one ofordinary skill in the art having access to the instant specification.

What is claimed is:
 1. A method of operating a wireless communicationsystem on a shared frequency spectrum, comprising: initializing a userequipment (UE) on a primary serving cell (PCell) on a licensed frequencyspectrum; configuring the UE to communicate with a secondary servingcell (SCell) operating on a carrier in the shared frequency spectrum;monitoring the shared frequency spectrum by a base station (eNB) todetermine if it is BUSY; transmitting to the UE on the shared frequencyspectrum if it is not BUSY; waiting for a first time period if it isBUSY; and directing the UE to vacate the carrier in the shared frequencyspectrum if it is BUSY after the first time period.
 2. The method ofclaim 1, wherein the shared frequency spectrum is an unlicensedfrequency spectrum.
 3. The method of claim 1, comprising accessing theshared frequency spectrum with carrier sense multiple access withcollision avoidance (CSMA/CA).
 4. The method of claim 1, comprisingdirecting the UE to vacate the shared frequency spectrum after the firsttime period by radio resource control (RRC) signaling.
 5. The method ofclaim 1, comprising directing the UE to deactivate the SCell after thefirst time period by Medium Access Control (MAC) signaling.
 6. Themethod of claim 1, comprising: transmitting to the UE on a firstplurality of channels if the shared frequency spectrum is not BUSY; andexcluding a second plurality of channels from the shared frequencyspectrum.
 7. The method of claim 1, comprising excluding a physicalbroadcast channel (PBCH) from transmissions on the shared frequencyspectrum.
 8. The method of claim 1, comprising transmitting a discoveryreference signal by the eNB on the shared frequency spectrum when it isBUSY.
 9. The method of claim 1, comprising transmitting a trackingreference signal by the eNB on the shared frequency spectrum.
 10. Amethod of operating a communication system on a shared frequencyspectrum, comprising: initializing a user equipment (UE) on a primaryserving cell (PCell) operating on a licensed frequency spectrum;transmitting data from the UE to at least one secondary serving cell(SCell) operating on the shared frequency spectrum; monitoring the atleast one SCell by the UE to determine a BUSY status; transmitting to abase station on the at least one SCell if it is not BUSY; and reportingthe BUSY status of the at least one SCell to the base station if it isBUSY.
 11. The method of claim 10, wherein the shared frequency spectrumis an unlicensed frequency spectrum.
 12. The method of claim 10,comprising accessing the shared frequency spectrum with carrier sensemultiple access with collision avoidance (CSMA/CA).
 13. The method ofclaim 10, comprising reporting the BUSY status in an uplink controlinformation packet on a Physical Uplink Control Channel of the PCell.14. The method of claim 10, wherein the BUSY status is transmitted on aPhysical Uplink Control Channel resource that is semi-staticallyconfigured by radio resource control signaling from the base station.15. The method of claim 10, wherein the BUSY status is transmitted on aPhysical Uplink Control Channel resource that is dynamically signaled bya downlink control information packet from the base station.
 16. Themethod of claim 10, comprising: determining a status of the at least oneSCell is BUSY; determining an IDLE status of at least another secondaryserving cell; and transmitting an identity of the at least anothersecondary serving cell on a Physical Uplink Control Channel of the PCellto the base station.
 17. A method of operating a wireless communicationsystem on a shared frequency spectrum, comprising: initializing a userequipment (UE) on a primary serving cell on a licensed frequencyspectrum; configuring the UE to communicate with a secondary servingcell (SCell) on the shared frequency spectrum; and receiving a radioresource management (RRM) report of the SCell from the UE if a primaryuser is detected on the shared frequency spectrum.
 18. The method ofclaim 17, wherein the shared frequency spectrum is an authorized sharedaccess (ASA) frequency spectrum, and wherein the primary user has ahigher access priority than the SCell.
 19. The method of claim 17,wherein the RRM report is periodic.
 20. The method of claim 17, whereinthe RRM report indicates a dynamic frequency selection (DFS) event.